To the Editor.—This letter is in regard to the treatment of patients with Parkinson's disease who are non-responsive to levodopa. For the past six months, six patients with Parkinson's disease who have been unresponsive to levodopa in doses of 6 to 8 gm per day for a period of 6 to 12 months have been studied. These patients are somewhat unique in that they have had no therapeutic response to levodopa nor have they had any dyskinesias due to this longterm levodopa therapy.These patients have been studied, and their response to levodopa alone was compared to their clinical response to levodopa plus MK 486, a Dopa decarboxylase inhibitor. During the trial of levodopa and the trial of MK 486 plus levodopa, blood levels of levodopa were determined, as were the homovanillic acid levels in the lumbar cerebrospinal fluid (CSF). These six patients had no clinical response to the